Anyone who has ever eaten an
apple has probably noticed that by the time you’re nearly finished and come back around to where you took your
first bite, that part of
the apple has started to turn brown. Why? Toasting bread turns it brown; frying
potatoes turns them brown; a few days after
Hallowe’en your
pumpkin will start to turn brown. Why?
It turns out, there’s a number of reasons why foods turn brown:
Caramelization
Caramelization occurs when a
sugar is heated to a high
temperature, over 248°F (120°C). At these temperatures the sugar undergoes a process called
thermal degradation and breaks down into two compounds: one, which provides the
caramel aroma, and a
residue that creates the familiar
brown colouring.
The process of
camelizing sugar is complex and there are a number of different flavours and colours that can be produced depending on the
acidity of the sugar compound, the heat used and the duration the sugar is heated.
Maillard Reactions
This reaction is named after the
French scientist,
Louis Camille Maillard, who discovered it in
1912. It occurs when a
carbohydrate (such as sugar) reacts with a
protein at high temperatures. More specifically, the
aldehydes in the sugar
molecule react with the
nitrogen (
amino acids) in the protein, to produce
melanoids, which in turn produce the flavours associated with the process.
Enzymatic browning
And now we come back to the
apple. Enzymatic browning, or
oxidation, doesn’t require heat and generally isn’t used to enhance the flavour of food, although the flavour of some foods like
tea and
coffee are enhanced by controlled enzymatic browning.
This type of browning occurs in
fruits like apples and bananas,
vegetables like potatoes, and
seafood like shrimp. A chemical reaction takes place between the enzymes (
polyphenol oxidase or
tyrosinase) in the food and the oxygen in the air, producing
melanins. The process damages the cells of the food and changes the colour and flavour.
In fruit and vegetables, the skin of the food prevents oxidation but it will brown quickly once cut. When cut, the damaged cells release the enzymes and react with the air.
Bruised fruit will also turn brown since the damaged cells in the bruise will react with the air in the food.
Removing the enzymes before they have a chance to
react with the air can control enzymatic browning. For example,
blanching (boiling in water for a short time) will prevent fresh shrimp from browning.
The reaction can also be prevented by not allowing the food to come in contact with oxygen. Ever notice that when
your Mom is cutting potatoes, she puts them directly into a pot of water rather than letting them sit in an open bowl? There’s less oxygen in the water. Apples can be stored for months at a time by flooding the storage container with
carbon dioxide and displacing the
oxygen.
Carmelization and
Maillard Reactions are also referred to as non-enzymatic browning.
Sources
http://www.scienceyear.com/outthere/index.html?page=/outthere/diner/why/browning.html
http://www.geocities.com/perfectapple/brown.html
http://www.exploratorium.edu/cooking/candy/caramels-story.html
http://www.agsci.ubc.ca/courses/fnh/410/colour/3_81.htm